Democratic presidential hopeful Julián Castro also criticized the Republican leader, saying that if he were elected president, he would make an appointment when the Senate was in recess if his nominee was not considered. Recess appointments are eventually considered by the Senate.

"We’ve known all along how hypocritical the @senatemajldr is. But his shamelessness at stealing a Supreme Court seat is appalling," Castro wrote.

"As President, I will work with the entire Senate to get my appointments confirmed, but won’t hesitate to make a recess appointment to the Court if the Senate refuses to consider my nominee," he added.

The composition of the Supreme Court has become increasingly discussed in recent weeks as a series of states passed laws restricting abortion rights. One, in Alabama, bans almost all abortions and could pose a direct legal challenge to the 1973 Roe v. Wade decision that legalized abortion nationwide.